Method and device for real time estimation of the applied pressure and of noisiness in a brake element, in particular a brake pad

ABSTRACT

A brake element is sensorized by at least one piezoceramic sensor arranged between a metallic support element and a block of friction material of a brake element, the sensor being completely embedded within the block. An electrical voltage signal generated by at least one piezoceramic sensor, without the need for a power supply, is picked up by an electrical circuit integrated into the metallic support element. The electrical voltage signal is processed in the form of equal length of samples per unit of time of the detected signal by successively processing in real time each sample of equal length of time sample of the signal by applying an algorithm. The algorithm is selected from at least one of a sequence of integrations of voltage values in the sample carried out in an interval of time in the order of milliseconds; FFT voltage data sample; and integral of the voltage data sample.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 U.S.C. §371 ofInternational Application No. PCT/IB2014/060778, filed Apr. 16, 2014,which claims priority of Italian Patent Application No. TO2013A000307,filed Apr. 17, 2013, the entire contents of each application beingherein incorporated by reference.

TECHNICAL FIELD

The present invention relates to a method and a device for the real timeestimation of the applied pressure and more particularly thedistribution of such pressure, together with noisiness present within abrake element, in particular a brake pad, by means of the directdetection of the forces acting upon said brake element when in useduring the braking operation of a vehicle.

TECHNICAL BACKGROUND

With the vehicle braking systems currently in production there is noway, other than the use of extremely complex external systems, todetect, when in use on a vehicle, the forces that are exchanged in usebetween the brake pads (or the brake shoes for vehicles still equippedwith drum brakes) and the element to be braked, the disk or drum, whichis bound to the wheel.

Normally two brake pads per vehicle disc (and therefore per wheel) arecontrolled by a mechanical caliper which is actuated by a hydraulicand/or electrical circuit when using the vehicle and that exertspressure on the pads that is proportional to the force applied to thebrake pedal. As pressure (nominal) is exerted by the caliper, the padsare pushed against the disc exerting a force that opposes the rotationof the disc, thus braking the vehicle.

The varying pressure distribution that results during the brakingprocess causes unwanted brake pad movements within the caliper itselfthus generating different phenomena known in the field of brake pads as,for example, residual torque, vibration and noisiness.

The presence of imperfections within the braking system for example, duein most cases to the wear of the mechanical parts employed (inparticular the disc, caliper and brake pads), lead to contact betweenthe surfaces designed to perform the braking which over time isnon-homogeneous. Above all, as a result of heavy braking, it may occurthat contact remains between the pads and the disc even after the footis lifted away from the brake pedal, thus resulting in the phenomenoncalled “residual torque”. Residual torque can also be generated by theformation of lobes on the disc, which may already be present when firstmounted or that form as a result of excessive disc overheating.

With respect to noisiness, this can be generated at differentfrequencies within the range of 0.1 and 20,000 Hz.

It is not currently possible to detect the pressure distribution otherthan in a static manner using pressure-sensitive paper or else by meansof systems that are directly interfaced with computers, such as forexample the Tekscan I-Scan® pressure mapping system(http://www.tekscan.com/brake-pad-pressure-distribution).

Neither is it possible to detect the type of noisiness, if not withcomplex equipment such as microphones and accelerometers positioned asnecessary within the braking system; whether the braking system ismounted on a chassis dynamometer or else on the vehicle.

This means that, with respect to issues arising from the non perfectfunctioning of the brake caliper rather than the pad or disc, currentvehicle braking systems are “blind”.

EP1531110 and GB2478423, which describe vehicle braking systems whereinpiezoelectric sensors are arranged, respectively, on the brake disc orbetween the piston and backplate of the brake calipers (the “backplate”is the metallic support element of the friction material of the brakepad that constitutes the carrier element) for the purpose to,respectively, produce a signal that is employed by an electric motor toadjust the position of the brake caliper piston or else to detect anysigns of wheel locking during braking, do not solve this problem.

EP1431606B1 describes a method for the measurement of forces applied toa layer of friction material wherein a functional layer, whoseelectrical resistance varies as a function of the forces applied to it,is associated with said layer of friction material; the variation in theelectrical resistance of the functional layer is then measured and is inproportion to the magnitude of the applied forces.

EP1923592B1 instead describes a brake or friction element having afriction layer and a support plate with at least one capacitive sensorarranged between the friction layer and the support plate, thecapacitance of which varies as a function of the force applied to saidfriction layer.

US2006/0254868 describes a system similar to that of EP1431606B1,wherein the variation in the electrical resistance of a brake elementlayer of friction material, such as a brake pad, is measured directly.

Even the systems described in EP 1431606B1, EP1923592B1 andUS2006/0254868 do not solve the above mentioned technical problem, sincethey are based on the electrical capacity or electrical resistancevariation of a sensor or of an entire functional layer, that allow withrelative precision for only static, or very slow, detection of theforces applied to the brake pad, but that are not capable of detectingrapid force variations such as those that occur during braking.

Moreover, these systems need to be continuously supplied electrically,which entails drawbacks, such as the relative amount of energy consumedand the considerable constructive complications associated with ensuringthe supply of electrical power between parts that are in relative rapidmotion.

DISCLOSURE OF INVENTION

The purpose of the present invention is to provide a method and a devicefor the real time determination of the applied pressure and noisinesspresent within a brake element, in particular a brake pad, in a simpleand economic manner, detecting in real time the presence and/or themagnitude of the stresses at the interface between the brake element(pad or shoe) and the element being braked (disc or drum brake) duringbraking.

It is also a preferred purpose of the invention to allow for thecontinuous measurement of the distribution of the contact pressuresacting upon the element being braked and the coefficient of frictionbetween the brake element and the element being braked, for example,between the brake pad and the disc.

The invention therefore relates to a method for the real timedetermination of the applied pressure and noisiness present in a brakeelement, in particular a brake pad. The invention also relates to adevice designed to perform the above method.

Here and below it is to be understood that the term “real time” meansthat the sequence of results provided by the system (detection andprocessing of desired parameters such as applied pressure, the presenceof noisiness and the friction coefficient) is fast enough to allow thebraking system to be acted upon while the phenomena that generate saidmeasured parameters are still running.

The solution according to the invention involves the insertion betweenthe support element of the block of friction material of the brakeelement, in particular defined by the backplate of a brake pad, and theblock of friction material itself, of sensors defined by piezoceramicelements in an suitable configuration, for example, distributed in adiscrete manner and preferably according to a symmetrical matrix, overthe whole extension of the interface between the support element and theblock of friction material, so as to allow in use for the timelydetection of the pressure applied and the distribution of such pressure.The piezoceramic components (sensors), that transduce the mechanicalenergy into electrical energy, are capable of generating an electricalsignal, a voltage for example, without the need for electrical power,and of transmitting said signal to the caliper that controls the brakepads rather than to the electronic control unit of the vehicle thatcontrols the dynamics of the vehicle and of the caliper itself. Theinformation is then post-processed by means of appropriate algorithms inorder to obtain the final desired information such as the pressuredistribution and its relative pressure centre together with a definitionof the type of noisiness, the presence of residual torque and thefriction coefficient value.

For example, using a brake pad with integrated piezoceramic sensors,when such a pad is connected to a suitable data processing system, it ispossible to determine the type of vibration and/or noisiness present,without the use of additional microphones and/or accelerometers.

According to the invention, this is achieved through the processing ofthe electrical signal by means of a Fast Fourier Transform (FFT) and/orby integrating the detected voltage values present in samples of equallengths of time of the voltage signals generated by the sensors withinone unit of time.

Surprisingly, the signal obtained by means of the real-time processingof the FFT of the electrical signal generated by each piezoceramicsensor, corresponds to the frequency of the noisiness that can bedetected, on the bench or by means of instrumentation on board of thevehicle. Among the types of noisiness that can be detected it ispossible to identify for example the “squeal” and the “creep-groan”,together with vibrations that are produced not only by the brake systembut also by the vehicle suspension system or bench test process itself.It should be noted that, according to a meaning generally attributed tothese terms by technical experts in the field, “squeal” is a noisinesscaused by vibrations induced by forces of friction, in which the brakedisc vibration modes are coupled to those of the brake pad frictionmaterial shoes themselves; “creep groan” is a high-intensity,low-frequency noisiness generated by the vibrations that affect roadvehicles at very low speeds.

It is also possible to determine which sensor, and hence the brake padposition, that generates noisiness, in order for example, to be able toeffectively evaluate by means of bench tests the effect thatchamfers/cuts have on the brake pad and/or any relief that might beimplemented in order to obtain an anti-noise effect.

With respect to residual torque, it is possible to determine itsmagnitude by integrating the voltage data in real time, in particular byintegrating the equal length of time signal samples coming from thesensors.

Similarly, by employing at least one piezoelectric sensor that iscapable of detecting shear forces, using the values obtained from theresulting rough voltage data and/or from the integral of the signalitself, the friction coefficient may then be determined by means of theknown relationship:

μ=FT/FN  (1)

where μ is the target friction coefficient, FT is the tangential forceacting at the interface between the brake pad and the element beingbraked (in this case tangentially to the brake disc) and FN is the forcenormal to the brake disc.

Given that the piezoelectric sensor signal is also proportional to thepressure exerted and the rigidity of the system, then using anappropriate mathematical relationship, to be explained later, it ispossible to employ the piezoelectric sensors as indirect wear detectors.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention willbecome clear from the following description of an exemplary non-limitingembodiment given purely by way of example and with reference to thefigures within the accompanying drawings, wherein:

FIG. 1 is a schematic view in elevation and partially according to alongitudinal sectional view taken along the wheel axle of a vehiclebraking system equipped with a device according to the invention;

FIG. 2 illustrates schematically an exploded perspective view and inaccordance with its construction sequence, a brake element, in this casea brake pad, which constitutes an essential element of the device inFIG. 1 according to the invention;

FIGS. 3, 4, 7 and 8 illustrate respective comparative diagrams of thesignals that are detected and processed in accordance with the method ofthe invention using the device illustrated in FIGS. 1 and 2;

FIG. 5 illustrates an example diagram regarding a calculationmethodology according to the method of the invention; and

FIG. 6 illustrates schematically and on an enlarged scale, a detail ofthe brake element components of FIG. 2.

PREFERRED EMBODIMENT OF THE INVENTION

With reference to FIGS. 1, 2 and 6, a sensorized vehicle brake elementis depicted, wholly indicated by reference number 1 and morespecifically, a brake pad in the example illustrated, which is intendedto equip a braking system 2 (FIG. 1), provided with a disc 3 (disc brakesystem); a disc 3 is attached as known in the art to each wheel of thevehicle (not shown for simplicity) and therefore rotates around a wheelaxis A.

The braking system 2 includes, in addition to disc 3 (and therefore foreach wheel of the vehicle) and per each disc 3, a brake caliper 4equipped with an actuator 5, known in the art, and a pair of brake pads1 carried by the brake caliper 4 together with the actuator 5, pads 1that in use can be pressed against disc 3 by means of the actuator 5 inorder to brake the rotation of disc 3 and with it the wheel to which itis attached and, as a result, the vehicle equipped with the brakingsystem 2.

Here and below the term “in use” means that the brake element is in anoperational condition, for example mounted within a braking systemcarried by a vehicle or else by a test bench, regardless of whether thebraking system is activated or not and that, therefore, the brakingsystem presses the brake element against the element to be braked.

The actuator 5 which is illustrated in a non-limiting embodiment is ofthe hydraulic type, equipped with a piston 6 that slides within an oilfilled chamber 7 that is pressurized by means of a pipe 8 when a userpresses the vehicle's brake pedal, and that more generally, iscontrolled by a vehicle control unit 10. Obviously, the actuator 5 mayalso be of the electrical type in which case it will be directlycontrolled by the on-board control unit 10. It is also clear that thecontent described hereinafter also applies to combinedhydraulic-electric systems.

Both here and below specific reference will be made to a brake elementconsisting of a brake pad 1, but it is clear that what will be said isalso identically applicable to the brake shoe of a drum brake, so thatthe braking system 2 described above can also be of a mixed typeconsisting of two discs 3 only (for example for the front wheels of thevehicle) and two brake drums (known in the art and not shown forsimplicity) for the rear wheels, that in use operate with brake elementsconsisting of brake shoes rather than with brake pads 1.

With further reference to FIG. 2, the brake elements 1 form part of adevice for determining in real-time onboard the vehicle the appliedpressure, and noisiness generated within each brake element 1, whichdevice is represented as a whole in FIG. 1 by the reference number 100.

For this purpose the brake elements 1 are designed as shownschematically in FIG. 2, wherein a brake element 1 is illustrated inexploded configuration in order to highlight its production process,which will be discussed thereafter.

Consequently, according to one aspect of the invention, brake elements 1include, besides a metallic support element 11, a heat insulating anddamping layer 12C, known as an “underlayer”, and a block 14 of frictionmaterial, all known in the art, at least one piezoceramic sensor 15,indicated schematically in FIG. 1 as an additional layer only which issandwiched between the block 14 of friction material and the metallicsupport element 11.

In particular, the damping and heat insulating layer 12C is positionedabove a first surface 13 of the metallic support element 11 which in useis intended to be facing an element to be braked, in the caseillustrated the disc 3, of a vehicle, and the block 14 of frictionmaterial is held rigid by the metallic support element 11 on the side ofthe surface 13 and above the damping/insulating layer or underlayer 12C.

In the case of brake pads, as in the non-limiting embodimentillustrated, the element 11, otherwise known as a “backplate”, is shapedlike a flat plate with a shaped contour. It is to be understood that inthe case of a brake shoe there will be elements corresponding to thosedescribed herein for the brake pad 1 for which, for those skilled in theart, the following description is easily transferable such thatsensorized brake shoes can also be constructed.

As it shall be seen, the at least one sensor 15 allows for the detectionof the contact forces between the pads 1 and the disc 3 when in use. Infact, the at least one piezoceramic sensor 15 converts receivedmechanical energy, in the form for example of force or pressure, intoelectrical energy without the need of a power supply. When constrainedto vary its thickness in the direction of said force, it consequentlygenerates a potential difference, i.e., an electrical voltage signal orelectric charge which varies according to the magnitude of the appliedforce.

In the preferred non-limiting embodiment illustrated, each brake pad 1comprises not only one sensor 15, but a plurality of piezoceramicsensors 15, which are directly integrated with the surface 13 and thatare spaced apart, preferably arranged according to a symmetrical array;further, sensors 15 can be individually activated.

The piezoceramic sensors 15 employed according to the invention differfrom known piezoceramic accelerometers in that they are without seismicmass, being formed of the active element only. Moreover, according tothe invention, the sensors 15 employed must be designed to have a signalresponse in the acoustic field from 20 Hz to 20,000 Hz, and to this endfeature a response time (the interval between subjecting the sensor tomechanical stress and the subsequent generation by the sensor of theelectrical signal) equal to 25 microseconds or less. In order to obtainoptimal signal accuracy, it is preferred that the piezoceramic sensorsbe chosen in such a way as to present a response time, as defined above,equal to or less than 0.16 microseconds.

The at least one piezoceramic sensor 15, or the plurality of sensors 15is/are arranged between the block 14 of friction material and themetallic support element 11 of the brake element 1, completely embeddedwithin the block 14 of friction material and directly rigidly supportedby the metallic support element 11, upon the surface 13 of the metallicsupport element 11 which is covered by the block 14 of friction materialthat in use faces the element to be braked 3.

In particular, the at least one sensor/plurality of sensors 15 is/areintegrated directly onto the surface 13 of the metallic support element11 before constructing the brake pad 1 in its entirety and thereforebefore forming, using techniques known in the art, the heat insulatingand damping layer 12C and the block 14 of friction material. Also,before forming the heat insulating/damping layer 12C, an additionalcontinuous or discontinuous electrically insulating layer 12B (FIG. 2)within which the at least one sensor 15/plurality of sensors 15 remainscompletely encased without the creation of any air bubbles, isimplemented upon surface 13.

Upon the electrically insulating layer 12B, in a manner known in theart, the layer 12C and the block 14 of friction material are thenconstructed in such a way as to completely cover the sensor(s) 15arranged on the surface 13, such that the block 14 becomes integral inone piece with the backplate 11, and in such a way that the at least onesensor 15/plurality of sensors 15 remains embedded within the block 14of friction material with the interposition of the damping and heatinsulating layer 12C, directly incorporated within the latter, with theonly interposition of the relative electrically insulating layer 12B.When there is only one sensor 15 it may occupy only a limitedposition/portion of the surface 13, or else it can be implemented in theform of a ceramic film that covers all or only a part the surface 13.

For simplicity, layers 12B and 12C are schematically illustrated in FIG.1 in the form of a single layer, indicated by the number 12.

In a preferred embodiment, the plurality of piezoceramic sensors 15consists of pressure sensors which are spaced apart so as to occupy theentire surface 13, but only in a discrete manner, i.e., incorrespondence to predetermined points/limited portions of the same.

The sensors 15 can be chosen from among commercial types provided thatthey are of a thickness, measured perpendicularly to the surface 13,equal to or less than that of the damping layer or underlayer 12C andare attached to the surface 13 by gluing for example or else by othertechniques known in the art.

Alternatively, the sensors 15 can be formed directly in situ onto thesupport element or backplate 11, thus integrating them directly with thesurface 13, integrally binding them to said surface 13, e.g. bysintering, as schematically illustrated in FIG. 2, for example by meansof a laser beam 30. As regards the material used to integrate thepiezoceramic sensors 15, a “soft” or “hard” type of PZT (lead zirconatetitanate) compound can be used for example or else a bismuth sodiumtitanate compound or modified lead metaniobate. The list of possiblematerials provided herein is not exhaustive and any piezoceramicmaterial currently known in the art or that may be available in thefuture and that meets the above requirements may be used in the presentinvention.

The brake pad 1 also includes an electrical circuit 18 shown in FIG. 2only in a schematic way and without any relation to reality.

With reference to FIG. 6 each piezoceramic sensor 15 comprises apiezoceramic block 115 made of a piezoceramic material having a Curietemperature greater than 200° C., provided with a couple of electricalconnections formed by electrodes 16 of opposing polarities, which areprovided onto opposite faces of the piezoceramic block 115 and that areconnected in any suitable manner, not shown in detail for simplicity, tothe electrical circuit 18, which is integrated with the metallic supportelement 11 of the brake element 1.

In the preferred embodiment illustrated, between the block 14 offriction material and the metallic support element 11 of each brakepad/brake element 1, a plurality of first piezoceramic sensors 15 barranged such that they are spaced apart in order to discreetly occupyall of the first surface 13 of the metallic support element 11 which iscovered by the block 14 of friction material and at least a secondpiezoceramic sensor 15 c arranged among the first piezoceramic sensors15 b, are arranged upon the surface 13 and completely embedded withinthe block 14 of friction material.

One of the sensors 15 b is schematically illustrated in FIG. 6. Thesensors 15 b have a flattened cylindrical shape and are formed by acylindrical block of piezoceramic material 115 b and by two electrodes16 b which entirely cover the respective base faces 116 b of the block115 b. Conversely, the sensor 15 c, also schematically illustrated inFIG. 6, has a flattened parallelepiped shape and consists of aparallepiped block of piezoceramic material 115 c and of two electricsignal connections defined by electrodes 16 c which entirely coverrespective base faces 116 c of the block 115 c.

Piezoceramic sensors are not of course made of materials havingpiezoelectric properties, but only of materials which, thanks to theircrystalline structure take on piezoelectric properties after appropriatepolarization process.

The sensors 15 b are thus polarized in a direction perpendicular to thesurface 13 and to the respective faces 116 b which are provided with theelectrodes 16 b; whereas the at least one piezoceramic sensor 15 c ispolarized in a direction parallel to the surface 13 and to therespective faces 116 c having the electrodes 16 c, in particular,perpendicularly to respective faces 117 illustrated in FIG. 6 by dashedlines, which faces, during the sensor 15 c production process, arecoated with polarization electrodes 118 (schematically illustrated, bydashed lines in exploded view, in FIG. 6), which are then removed.

In this way, the first piezoceramic sensors 15 b are designed togenerate a voltage signal in response to the application of stressparallel to the direction in use of the pressure applied to the brakeelement 1, while the at least one second piezoceramic sensor 15 c isdesigned to generate a voltage signal in response to the application ofstress transverse to the direction in use of the pressure applied to thebrake element 1, in particular tangentially to the disc 3 and therelative axis of rotation A.

The electrical circuit 18 is connected to an electrical connector 20, inthe non-limiting example illustrated by means of a cable 19, which canhowever be omitted by directly making the connector 20 an integral partof the metallic support element 11. The connector 20 and the electricalcircuit 18 are constructed in such a way as to be not only designed toreceive and transmit the electrical signals generated and transmitted inuse by the sensor/sensors 15, but also to be connected to a voltagegenerator 25 (FIG. 2), for example at the end of the pad 1 manufacturingprocess for the purpose of biasing and/or re-polarizing the sensor(s)15, which, otherwise, could remain inert or not fully functioning.

The device 100 according to the invention also comprises a processingmeans 22 which is for example connected in a detachable way to theelectrical circuit 18 and to the at least one piezoceramic sensor 15 (tothe plurality of sensors 15 b and 15 c) by means of the connector 20 andthe optional electrical cable 19 of each brake pad 1. The processingmeans 22 may also be integrated into the connector 20 and will in anycase be connected to the control unit 10 that controls the actuators 5which, in use, are intended to push the block 14 of friction material ofeach brake element 1 against the brake element defined by the disc 3 (orby a drum brake in the case of drum brakes).

According to a further aspect of the invention, the brake pad 1 alsoincludes, in addition to a single piezoceramic sensor 15 or a pluralityof piezoceramic sensors 15 b and 15 c, a temperature sensor 21 of anyknown type, which is also integrated into the backplate 11 in a similarway to the sensors 15 and that is electrically connected to theelectrical circuit 18, and that in use also transmits electrical signals(e.g. a voltage) to the connector 20.

The electrical circuit 18 and the connector 20 (together with theoptional cable 19) are made in order to provide the processing means 22with separate signals for each sensor 15 and 21.

Each brake element 1 according to the invention comprises, therefore, inaddition to the components already described:

means, represented by the connector 20 (and the optional cable 19) topick-up from the at least one piezoceramic sensor 15/from the sensors 15b, 15 c (and the sensor 21 when present) and by means of the circuit 18,a respective electrical voltage signal generated by the sensor/s,without the need for a separate power supply, in response to theapplication to each sensor 15/15 b, 15 c of a mechanical stressresulting from contact between the brake element 1 and the element to bebraked 3;

first means 121, represented by a block drawn with dashed lines in FIG.1, for the real time processing of the electric voltage signal generatedby the at least one piezoceramic sensor 15/by the sensors 15 b, 15 c inorder to generate equal length of time samples of said signal, in otherwords, segmenting the signal into sequences taken at constant intervalsof time;

second means 122, also represented by a block drawn with dashed lines inFIG. 1, for processing in real time each of the equal length of timesamples of the signal generated by at least one piezoceramic sensor15/by the sensors 15 b, 15 c by applying an appropriate algorithm tosaid equal length of time samples of the signal.

According to an aspect of the invention, this algorithm is selected froma group consisting of: a sequence of integrations of the voltage valuespresent within the equal length of time samples of the signal, eachintegration being carried out in an interval of time in the order ofmilliseconds; FFT (Fast Fourier Transform) of the voltages in the sampleof equal length of time of the signal; integral of the voltages in thesamples of equal length of time of the signal; any combination thereof.

The processing means 121 and 122 may be integrated into the processingmeans 22, as schematically illustrated in FIG. 1, or else implementedseparately or as an integral part, in the form of hardware or software,of the control unit 10.

The device 100 according to the invention, in addition to the processingmeans 22 which is integrated, or linked to, the processing means 121 and122, and the sensor 15/sensors 15 b, 15 c incorporated into each brakeelement 1, also comprises signalling means 29 that can be activated bythe processing means 22 or by the control unit 10 in response to anelectrical signal processed by the processing means 121 and 122, as itwill be seen below.

By means of the device 100 described, and in particular thanks to thesuitably sensorized brake elements 1 as already described, according tothe invention there can be implemented, both on the vehicle during itsnormal use, and therefore during all vehicle braking phases, and duringbraking system 2 bench tests, a method for the real time estimation ofthe applied pressure and noisiness present in a brake element, such asthe brake element 1 (in this case the brake pad) a method which isillustrated within the experimental diagrams of FIGS. 3, 4, 7 and 8,diagrams that were obtained from bench tests carried out on brake pad 1prototypes equipped with multiple sensors 15 b together with a singlesensor 15 c as described earlier, and which involves the steps listedbelow.

A first step of the method of the invention involves having at least onepiezoceramic sensor 15 or 15 b, 15 c between the block 14 of frictionmaterial and the metallic support element 11 of the brake element 1,that is completely embedded within the block 14 of friction material andintegrally supported directly by the metallic support element 11, uponthe surface 13 of the brake element 1 which is covered by the block 14of friction material and which in use faces towards the element to bebraked 3, as previously described in detail.

A second step of the method of the invention involves picking up fromthe at least one piezoceramic sensor 15/from sensors 15 b, 15 c, bymeans of the circuit 18 and with the brake element 1 in use, andtherefore with the element to be braked 3 in rotation around axis A, arespective electrical signal, in the example illustrated a voltage ST,whose trend over time is illustrated in the upper part of FIGS. 3, 4, 7,8. The ST signal is generated by the sensor 15/sensors 15 b, 15 c,without the need for a power supply, in response to the application of amechanical stress on the sensors 15 resulting from contact between thebrake element 1 and the element to be braked 3.

In the illustrated example, the sensorized brake pad 1 employed in theexperiments that generated the graphs of FIGS. 3, 4, 7, 8 was equippedwith four sensors 15 b (the signal of channels A, C, D and E) and onesensor 15 c (the signal of channel B). The graphs illustrated in FIGS.4, 7 and 8, each relate to complete braking events, the braking event inthe graph of FIG. 4 was carried out in an abrupt fashion, as evidencedby the initial peaks in the ST signal; the braking operation for thegraphs in FIGS. 7 and 8 were instead carried out gradually. The finalpeaks in the ST signal in FIGS. 4, 7 and 8 correspond to the end of thebraking operation and therefore the detachment of the brake element 1from the element being braked 3.

It should be noted that a simple examination of these graphs (andtherefore the mere understanding of the raw data) reveals for examplethat the first brake pad used during the braking operation in FIG. 4presents an abnormal contact at the sensor 15 b related to channel D,which is highlighted by the negative voltage peak. This informationalone, produced for example during the bench testing of a new brake pad,would allow for the design revision of said brake pad and/or its support(caliper) in order to avoid this problem when subsequently in use on avehicle. Or else where obtained from a vehicle in use, this informationcould be passed to the control unit 10, which could for example operatein a suitable manner one or more of the actuators 5 located on the brakecaliper.

The graph of FIG. 3 (upper section) relates to the ST signal produced bya single sensor 15 b (related to channel A) under different brake pad 1operating conditions, as shown in the legend in the lower part of thesame FIG. 3, namely when pad 1 is not in contact with the disc 3 andwhen pad 1 approaches the disc 3.

A third step of the method of the invention involves the real timeprocessing of the electrical voltage signal ST generated by thepiezoceramic sensor 15/sensors 15 b, 15 c so that equal length of timesamples are taken of the signal itself, for example for each second ofbraking, which sample is represented by all of the points of the curvesthat represent the ST signal within the unit of time underconsideration. In the discussed embodiment, a plurality of equal lengthof time samples of the signal ST is generated by the means 121throughout the duration of a braking operation (a few seconds).

According to an aspect of the invention, said signal sampling, carriedout in this case by the processing means 121, must involve a largenumber of points, that is of ST signal voltage values which are variablein time. In particular, this step is performed in order to collect aplurality of digital values of the ST signal using a sampling frequencyequal to or greater than twice of one highest target frequency containedwithin said ST signal; in order to optimally detect frequencies in theacoustic field a sampling frequency of at least 40 kHz is used (i.e.equal to or greater than 40 kHz), and preferably equal to 50 kHz. Inother words, this means that each group of digital values representingan equal length of time sample in which the ST signal processed by theprocessing means 121 is subdivided is composed of (i.e. contains) atleast 40,000 values of the ST signal per second and preferably 50,000values of the ST signal per second, which signal, as is evident from thegraphs of FIGS. 3, 4, 7, 8, is an oscillating signal, with voltagevalues that pass from positive to negative, due to the unavoidablevibrations that are generated between the brake element 1 and theelement being braked 3 during braking. The above values for the STsignal sampling rate (sampling frequency) are, as will be seen, criticalaccording to the invention for analyzing the noisiness associated withthe brake pad 1 during braking.

A fourth step of the method of the invention involves finally thefurther processing, again in real time, of each equal length of timesample of the voltage signal ST generated by the sensors 15, 15 b, 15 cby the application using means 122 of an appropriate algorithm appliedto each equal length of time sample of the signal processed by means121.

According to the invention, this algorithm is to be selected from agroup consisting of:

a sequence of integration of the voltage values contained within eachequal length of time sample of the signal ST of the plurality of equallength of time samples under consideration, each integrationrepresenting a “Microintegration” of the ST signal and being processedwithin a interval of time in the order of milliseconds;

an FFT (Fast Fourier Transform) of each equal length of time ST signalsample generated by each sensor 15, 15 b, 15 c and applied to theentirety of each equal length of time ST signal sample;

the comprehensive integral of the voltage values for each entire equallength of time ST signal sample generated by each sensor 15, 15 b, 15 c,and

any combination thereof.

The result of the application of these algorithms to the oscillating STvoltage signal generated by each piezoceramic sensor 15 is shown inFIGS. 3, 4, 7, 8.

In particular, the central part of FIG. 3 shows by way of example thegraph resulting from the “Microintegration” of two of the ST signalsshown in the upper part of the same figure. As is evident, the result isa series of curves C1-Cn; curve C1 corresponds to the rest state of thebrake pad 1 (no braking, the pad is not touching), curve C2 correspondsinstead to the presence of residual torque due to the fact that thebrake pad 1 is in contact with the disc 3 but with no braking pressurebeing applied.

Conversely, the graph in the lower part of FIG. 3 corresponds to theapplication to the ST signals of the upper part of the same figure of anFFT and illustrates also in this case the presence of any residualtorque generated.

The graphs in the middle part of FIGS. 4 and 7 correspond to the resultsobtained by the ST signal processing generated during the bench testingof a brake pad 1 and illustrated in the upper part of the same figuresby means of an FFT; the graphs in the middle part of FIGS. 4 and 7 bothshow peaks at specific frequencies. For comparison the lower part of thesame FIGS. 4 and 7 give the graphs from the bench results obtainedduring the same test using sophisticated microphone-based equipmentknown in the art, which illustrates the frequencies and intensity of thenoisiness generated in use by the brake pad 1.

Surprisingly, the graphs obtained using an FFT to process the equallength of time ST signal samples obtained, based on the criticalparameters (sampling frequency) indicated above, have the same trend asthe noisiness graphs obtained by means of microphone-based equipment;which graphs exhibit signal peaks at exactly the same frequency and thattherefore correspond to the generation of noisiness at that frequency,noisiness whose intensity is proportional to the height of the peak.This result has proved to be reproducible with absolute precision underall test conditions. Furthermore, since the signals are generated byeach separate sensor 15 b, 15 c, each associated with a specificchannel, it is also possible to detect which physical point on the brakepad 1 generates the noisiness detected, something which is impossible todo using the commonly used microphone-based equipment and that costsmuch more than the device 100 equipped with sensorized brake pads 1 oneach wheel of the vehicle, as according to the invention.

Finally, with reference to FIG. 8, at the bottom on the left of FIG. 8there are the graphs obtained by integrating the entire ST signal givenin the upper section of the same figure and shown separately for eachchannel. The sawtooth waveforms in graphs G1-G5 qualitatively correspondto the distribution of the pressure applied locally to the brake pad 1corresponding to the sensors 15 b, 15 c resulting from the contact withthe disc 3. The channel B graph, corresponding to the integration of theentire set of the equal length of time ST signal samples from sensor 15c, is inverted since it corresponds to the application of a tangentialload (pressure), which “stretches” the sensor 15 c, instead ofcompressing it, as is the case for the sensors 15 b.

On the basis of what has been described insofar, it can be deduced thatthe second step of the method of the invention is carried out byseparately picking up for each first sensor 15 b and second sensor 15 c,by means of the circuit 18, the relative electrical ST voltage signalgenerated by each sensor, and that the third step of the method of theinvention is carried out in real time by processing the electrical STvoltage signal generated by each first and second piezoceramic sensor 15b, 15 c in order to obtain equal length of time ST signal samplesgenerated separately for each sensor, preferably such as to read aplurality of digital values at a rate of at least 40,000 values persecond; while the fourth step of the method of the invention isperformed by applying to each equal length of time ST signal sampleobtained from each sensor 15 b, 15 c, an algorithm selected from thegroup of algorithms listed above in order to identify a specificphysical quantity of interest.

The method of the invention also includes a fifth step for each firstsensor 15 b regarding the processing of a curve (curves C1-C2 of FIG. 3)which represents the residual torque trend locally present in use withinthe brake element 1. Said fifth processing step is performed in realtime and involves the application to each equal length of time ST signalsample obtained from each piezoceramic sensor 15 b of an algorithmconsisting of an integration sequence of the voltage values read fromeach sensor, each integration being performed within an interval of timein the order of milliseconds.

The method of the invention also includes, in addition or as analternative to the previous fifth step, a sixth step of processing foreach first 15 b and second piezoceramic sensor 15 c a signal voltage vs.frequency (those shown by the graphs in the middle part of FIGS. 4 and7), wherein the presence of a peak at a given frequency represents thegeneration between the brake element 1 and the element to be braked 3 ofnoisiness having the same frequency, and an intensity that isproportional to the amplitude of the voltage signal; said sixth stepbeing carried out through the application in real time of an algorithm,consisting of an FFT (Fast Fourier Transform), to each of the equallength of time ST signal samples obtained from each piezoceramic sensor.

Finally, the method according to the invention also includes, inaddition or as an alternative to the previous fifth and sixth steps, theseventh step of processing for each sensor a curve representing, duringan interval of time equal to the execution of a full braking operation,the trend of the local contact pressures between the brake element andthe element to be braked for each first sensor 15 b and the tangentialforce applied between the brake element 1 and the element 3 to be brakedfor at least the second sensor 15 c; said seventh step being carried outby applying an algorithm in real time that calculates the comprehensiveintegral of each equal length of time ST signal sample obtained fromeach piezoceramic sensor 15 b, 15 c.

According to a further aspect of the invention, the method of theinvention also includes the step of calculating the friction coefficientvalue μ between the brake element 1 and element 3 to be braked during abraking event by calculating the ratio between the integral of the STvoltage data read by at least the second sensor 15 c and the integral ofthe ST voltage data read by at least one of the first sensors 15 b, byapplying to the invention the known relationship

μ=FT/FN.  (1)

According to another aspect of the invention, the method of theinvention also includes a step of arranging a temperature sensor 21,connected to the electrical circuit 18, upon the first surface 13between the block 14 of friction material and the metallic supportelement 11 which surface 13 is completely embedded within the block 14of friction material; and the step of correcting the ST signal voltagevalues obtained from the piezoceramic sensors 15 b, 15 c as a functionof the temperature detected by the temperature sensor, according to anempirical relationship known in advance and that can be stored by theprocessing means 122.

Similarly, the method of the invention may comprise:

a calibration step, wherein a selected type of brake element 1 equippedwith at least one piezoceramic sensor 15 is subjected to a bench testwherein, by means of measurement equipment external to the brake element1, known in the art and not illustrated for simplicity sake, at leastone brake element operating parameter is measured, selected from a groupconsisting of: the contact pressure between a brake element and anelement to be braked, the friction coefficient between a brake elementand an element to be braked and the residual torque; and wherein themeasured operating parameter is correlated with the processing result asaccording to the fourth step previously described and with the result ofthe correlation being parameterized in a table; and

a calculation step wherein the processing result according to the fourthstep previously described is compared with the table, suitably stored bystorage means 123 connected to the electrical circuit 18 (integrated forexample within the processing unit 122, FIG. 1) and the instantaneousbrake element 1 operating parameter value is supplied in real time whenin use during each braking operation.

In practice, by means of said processing step the control unit 10 of thevehicle can be able to recognize in use, second by second, the valuesand distribution of the pressures and shear stress applied to the brakepad 1 by the disc 3 during braking, the value for the instantaneouscoefficient of friction and the magnitude, frequency and location of anynoisiness generated during braking. The control unit 10 will then be ina position suitable to intervene in real time, for example, upon theactuator means 5, to correct any braking anomaly and to optimize saidbraking as a function of the driving conditions of the vehicle, asmonitored by other on-board systems, which can be placed in directcommunication with each other and with the device 100 described forexample by means of the CAN bus of the vehicle itself.

Finally, the method according to the invention also allows for theindirect detection of the brake element wear by means of the samepiezoceramic sensors 15 b, 15 c described above. To this end, the methodof the invention comprises a prolonged (over time) comparative step ofprocessing the electrical ST signal from at least one piezoceramicsensor 15 b which is performed in order to identify a time-based decayfunction associated with the e.g. linear relationship between the STelectrical signal and the pressure exerted at said sensor (detectablefor example by the hydraulic circuit 8, or directly by the control unit10) during all steps of the vehicle braking, and the step of comparingthe instantaneous decay function value with a threshold value, belowwhich the wear signalling means 29 of the brake element 1 is activated.

In other words, it exploits the relationship illustrated in FIG. 5,which represents the application of the following formulas:

Ki=Ki(W,T,t)  (2)

V=aK0+bKi*p  (3)

where V=the ST output signal magnitude (in volts), K0=the initial pad1/disc 3 braking system stiffness, Ki=the braking system stiffness attime “i”, P=applied pressure, W=brake pad wear T=temperature and t=time.

As a function of the pressure applied by the brake system, a brakesystem with greater stiffness will produce a greater response, andtherefore a higher ST voltage signal.

In conclusion, according to the method and the device of the inventionit is possible to assess in a quantitative and qualitative manner, theresidual torque, the wear and the noisiness and vibration intensitycoming from the braking system.

The purposes of the invention are thus fully achieved.

1. A method for the real time estimation of the applied pressure andnoisiness in a brake element, the method comprising the steps of:i)—providing at least one piezoceramic sensor between a frictionmaterial block and a metallic support element of the brake element, theat least one piezoceramic sensor being connected to an electric circuit;ii)—with the brake element in use, picking up from said at least onepiezoceramic sensor, by means of said circuit, a respective electricvoltage signal (ST) generated by the at least one piezoceramic sensor inresponse to the application of a mechanical stress on the at least onesensor; iii)—processing in real time the electric voltage signal (ST) bytaking equal length of time samples of said electric voltage signal; andiv)—processing in real time each said equal length of time samples ofsaid electric voltage signal by applying an algorithm selected from atleast one of the group consisting of: sequence of integrations of thevoltage values present in the sample, each integration being carried outin an interval of time in the order of milliseconds; FFT (Fast FourierTransform) of the voltages in the sample; or integral of the voltages inthe sample generated by each sensor.
 2. A method according to claim 1,wherein the real time electric voltage signal processing step isperformed so as to collect a plurality of digital values using asampling frequency equal to or higher than the double of a targetedhighest frequency contained in said electric voltage signal (ST).
 3. Amethod according to claim 2, wherein a sampling frequency of at least 40kHz is used.
 4. A method according to claim 1, wherein a plurality offirst piezoceramic sensors are arranged on said first surface, betweensaid friction material block and said metallic support element of thebrake element, said plurality of first piezoceramic sensors beingarranged spaced apart from one another so as to occupy in discretemanner the entire first surface of the metallic support element; and atleast one second piezoceramic sensor being also arranged on said firstsurface, spaced apart from the plurality of first piezoceramic sensors;the step ii) being performed by separately picking up for each first andsecond piezoceramic sensor, by means of said circuit, a respectiveelectric voltage signal (ST) generated by each piezoceramic sensor; thestep iii) being performed by processing in real time the electricvoltage signal (ST) generated by each first and second piezoceramicsensor so as to generate separately, per unit of time, equal length oftime samples of said signals for each piezoceramic sensor; and the stepiv) being performed by applying to each sample of said equal length oftime samples of said signals obtained by means of each piezoceramicsensor, an algorithm chosen from at least one of the group consistingof: sequence of integrations of voltage values detected for eachpiezoceramic sensor, each integration being carried out in an intervalof time in the order of milliseconds; FFT (Fast Fourier Transform) ofeach equal length of time sample of said signals generated by eachpiezoceramic sensor or; integral of the equal length of time samples ofsaid signals generated by each piezoceramic sensor.
 5. A methodaccording to claim 4, wherein the plurality of first piezoceramicsensors are biased in a direction perpendicular to the first surface,while the at least one second piezoceramic sensor is biased in adirection parallel to the first surface, so that the plurality of firstpiezoceramic sensors are adapted to generate said electric voltagesignal (ST) in response to the application of stresses parallel to adirection of application in use of an actuating pressure on the brakeelement, while the at least one second piezoceramic sensor is adapted togenerate said electric voltage signal (ST) in response to theapplication of stresses transversal to a direction of application in useof the actuating pressure on the brake element.
 6. A method according toclaim 5, wherein each of said first and second piezoceramic sensors areprovided with electric signal connections to said circuit carried byopposite first faces of a piezoceramic block belonging to eachpiezoceramic sensor arranged parallel to the first surface of themetallic support element of the brake element.
 7. A method according toclaim 4, further comprising the step of processing a curve (C1-C2) foreach of the plurality of the first piezoceramic which represents theresidual torque trend locally present in use on the brake element, saidstep being performed by applying an algorithm in real time to each equallength of time samples of said signals obtained by means of eachpiezoceramic sensor consisting in an integration sequence of thedetected voltage values by each piezoceramic sensor, each integrationbeing in an interval of time in the order of milliseconds.
 8. A methodaccording to claim 4, further comprising the step of processing a signalvoltage vs. frequency for each first and second piezoceramic sensor, inwhich signal the presence of a peak at a determined frequency representsthe generation of a noise between the brake element and an element to bebraked having the same frequency and intensity proportional to theamplitude of the voltage signal, said step being performed by applyingan algorithm consisting of an FFT (Fast Fourier Transform) of the equallength of time samples of said signals made in real time on each equallength of time samples of said signals obtained by means of eachpiezoceramic sensor.
 9. A method according to claim 4, furthercomprising the step of processing a curve (G1-G5) for each piezoceramicsensor which represents the trend of local contact pressures between thebrake element and an element to be braked during the interval of timeequal to the execution of a complete braking operation for each of theplurality of first piezoceramic sensors and of the tangential forceapplied between the brake element and element to be braked for the atleast one second piezoceramic sensor, said step being performed byapplying an algorithm consisting in running the overall integral of theequal length of time samples of said signals in real time on each equallength of time sample of said signals obtained by means of eachpiezoceramic sensor.
 10. A method according to claim 4, furthercomprising the step of processing the friction coefficient value presentbetween the brake element and an element to be braked during a brakingoperation by calculating the ratio between the integral of the value ofthe voltage data (ST) detected by the at least one second piezoceramicsensor and the value of the integral of the voltage data detected by atleast one of the plurality of first piezoceramic sensors.
 11. A methodaccording to claim 1, further comprising the step of arranging atemperature sensor connected to said electric circuit on said firstsurface; and the step of adjusting the values of the voltage signalsobtained from said at least one of the first and second piezoceramicsensors on the basis of the temperature detected by the temperaturesensor.
 12. A method according to claim 1, further comprising a step ofcalibrating, in which a selected type of brake element provided with atleast one piezoceramic sensor is subjected to a bench test in which atleast one operating parameter of the brake element is measured by meansof measuring means external to the brake element, the parameter beingchosen from at least one of the group consisting of: contact pressurebetween brake element and an element to be braked, friction coefficientbetween brake element and an element to be braked, or residual torque;and wherein the measured operating parameter is correlated with theresult of the processing according to step iv) and the result of thecorrelation parameterized in a table; and a step of calculating, inwhich the result of the processing according to step iv) is comparedwith the table, appropriately stored in storage means connected to saidelectric circuit and in which the instantaneous value of said operatingparameter of the brake element when in use during each braking operationis supplied in real time.
 13. A method according to claim 1, furthercomprising: a step of comparative processing protracted over time of theelectric voltage signal (ST) coming from the at least one piezoceramicsensor performed so as to identify a decay function over time of alinear ratio between the electric signal of the at least onepiezoceramic sensor and the pressure exerted at the at least onepiezoceramic sensor during the step of braking of the vehicle; and astep of comparing an instantaneous value of the processed decay functionwith a threshold value, below which wear signaling means of the brakeelement are activated.
 14. A method according to claim 1, wherein the atleast one piezoceramic sensor has a response time equal to or less than25 microseconds.
 15. A device for the real time estimation of theapplied pressure and of the noisiness in a brake element, the devicecomprising: i)—at least one piezoceramic sensor arranged between afriction material block (14) and a metallic support element of the brakeelement, the at least one piezoceramic sensor being connected to anelectric circuit; ii)—means for picking up from said at least onepiezoceramic sensor and by means of said circuit, a respective electricvoltage signal (ST) generated by the at least one piezoceramic sensor inresponse to the application on the at least one piezoceramic sensor of amechanical stress consequent to a contact between brake element and anelement to be braked; iii)—first means for processing the electricvoltage signal generated by said at least one piezoceramic sensor inreal time so as to generate equal length of time samples of saidsignals; iv)—second means for processing each equal length of timesamples of said signals generated by means of the at least onepiezoceramic sensor in real time by applying an algorithm to the equallength of time samples of said signals chosen from at least one of thegroup consisting of: sequence of integrations of the voltage valuespresent in the equal length of time samples of said signals each carriedout in an interval of time in the order of milliseconds; FFT (FastFourier Transform) of the equal length of time samples of said signals;or integral of the equal length of time samples of said signalsgenerated by each piezoceramic sensor.
 16. A device according to claim15, wherein a plurality of first piezoceramic sensors and at least onesecond piezoceramic sensor are arranged on said first surface, saidplurality of first piezoceramic sensors being arranged spaced apart fromone another so as to occupy in discrete manner the entire first surfaceof the metallic support element covered by a friction material block,and the at least one second piezoceramic sensor being arranged spacedapart from the plurality of first piezoceramic sensors; said electriccircuit being arranged on the metallic support element of the brakeelement and the plurality of first piezoceramic sensors being polarizedin a direction perpendicular to a first surface, while the at least onesecond piezoceramic sensor is polarized in a direction parallel to thefirst surface, so that the plurality of first piezoceramic sensors areadapted to generate said voltage signal in response to the applicationof stresses parallel to a direction of application in use of anactuating pressure on the brake element, while the at least one secondpiezoceramic sensor is adapted to generate said voltage signal inresponse to the application of stresses transversal to a direction ofapplication in use of the actuating pressure on the brake element.
 17. Amethod according to claim 1, wherein the braking element is a brake pad.18. A device according to claim 15, wherein the braking element is abrake pad.